Forced air ovens and furnaces such as those used in baking certain foods such as pizzas waste 90% or more of their heat input up the exhaust flue. Air is pulled in, heated, moved in a single pass over the product being baked, and exhausted immediately up the flue. The desirability of increasing the efficiency of these units has risen with the upward spiral of energy costs. Attempts have been made to conserve energy by recycling the heated air in multiple passes but this has required complicated and expensive machinery, and, when applied to pizza baking operations for example, have always resulted in a pizza product having degraded taste or texture.
In a typical pizza restaurant, the oven may be rated at a maximum of 70 kilowatts electrical power input. It will maintain a full oven baking temperature of 500.degree. F. with a constant average input of 33 kilowatts or 113,000 BTU per hour. About 5% is used in heating the room, 4% is absorbed by the product being baked, and about 91% is constantly lost up the stack. This is approximately 30 kilowatts or 102,400 BTU per hour lost, hour after hour, twelve hours a day.
That same, typical pizza restaurant is likely to have a 45 kilowatt electric water heater which will run approximately half the time with an average input of 23 kilowatts or 78,506 BTU per hour. Therefore, if only 77% of the waste heat going up the stack could be re-captured and used for water heating purposes, no primary energy would be needed.
Because of such substantial energy requirements just to generate the hot water needed in a pizza restaurant, if all or even a major part of the energy needed for hot water could be recovered from the stack, the cost saving would be astounding, amounting to many thousands of dollars per year in a single medium size restaurant operating only twelve hours per day.
Users of electrical energy for ovens of this type are charged by the power generating utility in two ways: first, for the actual amount of power consumed; and, second, for the peak load capacity which the power company has to maintain. Thus, cost savings could result in excess of the actual energy savings, simply by lowering the peak KW demand.
It has long been recognized by people skilled in this field that utilizing the waste flue heat to produce hot water for cleaning, cooking, heating and general washroom and restaurant use would be the answer to recovering some of this wasted energy. A problem never solved prior to the present invention has been how to limit or stop heat transfer from a high capacity oven or furnace, to the water in which the recovered heat is being transferred, when the storage tank or tanks are at their maximum temperatures and no further temperature increase would be safe or desirable at that particular time.
As a practical matter, 160.degree. F. is the maximum temperature that water should be heated in a coil, and stored in a tank at ordinary domestic tap water pressures, without causing localized boiling and lime deposition. Since the flue gases are always at much higher temperatures than the boiling point of water, simply stopping the flow in the heat recovery coil could allow the water in the coil to rise to dangerous temperatures and pressures.
Prior art systems have attemped to solve this problem in various ways. Farris U.S. Pat. No. 4,211,187 allows the water to boil out of the heat recovery coil into a holding tank. Estabrook U.S. Pat. No. 3,999,709 dissipates the excess heat through a secondary coil when the holding tank is at its predetermined maximum temperature. Pemberton et al U.S. Pat. No. 4,066,210 simply stops the circulatory flow and does not address the problem of overheating in the coil tubes. Borovina et al U.S. Pat. No. 3,896,992 and DeBoer U.S. Pat. No. 4,136,731 both have systems designed for very low output home-type furnaces. These home-type furnace systems do not adequately solve the problem of overheating in the heat recovery coil tubes; they simply rely on a thermal siphon method of holding down temperature which is inadequate for a high output oven with flue temperatures reaching 600.degree.-700.degree. F. Each of these prior art systems fail the test of practicality because of inefficiency, complexity, tendency for lime to build up, or the possibility of overheating or overpressure leading to rupture of a tube or tank. For these reasons, none of the systems developed in the past are practical for use on a high capacity oven.
Accordingly, this situation is in need of improvement.